Merge branch 'claude/dependency-notes': a library's AGENTS.consumer.md in its consumers' AGENTS.md

Brings in dependency notes: a crafter-build project that ships an
AGENTS.consumer.md beside its project.cpp has it kept in a marked
section of each consumer's AGENTS.md, refreshed by `crafter-build` and
`crafter-build test`, chosen by cfg.dependencyNotes (Transitive,
Direct, None). The new partition is Crafter.Build:Agents; the loader
records where each GitProject and LocalProject came from and what a
project file's pragmas resolved to.

The branch started before master's four external-fetch commits. Both
touch External.cpp, the Clang partition, project.cpp, README.md and
AGENTS.md, but in different places, so the merge needed no hand
reconciliation; the merged tree passes build.sh, the self-host build,
format --check and the test suite.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

interfaces/Crafter.Build-External.cppm

@@ -114,3 +114,30 @@ export namespace Crafter {
     // SHA for reproducible, offline-after-first-fetch builds.
     fs::path GitFetch(const GitSource& source);
 }
+
+namespace Crafter {
+    // Where GitProject or LocalProject loaded a Configuration from. The
+    // loader keeps this beside the cache that owns the Configuration, since
+    // the Configuration itself cannot say: only the one they return has its
+    // path rebased, and the configurations a dependency's project file makes
+    // for itself keep a path such as "./", which resolves against the
+    // consumer's directory.
+    struct LoadedProject {
+        fs::path projectFile;   // absolute
+        // What a consumer's AGENTS.md calls it: the repository's name for a
+        // git project, with the path below the clone's root for one that is
+        // not at it, and the directory's name for a local one. The same on
+        // every machine and for a --local checkout and its clone alike,
+        // which a path into the cache or a sibling checkout is not, and free
+        // of what a URL can carry, credentials included.
+        std::string name;
+        // Which library this is, whatever the revision: the repository and
+        // the project file in it for git, the project file for a local one.
+        // Two specs that pin one library differently are one library.
+        std::string identity;
+    };
+    // The LoadedProject behind a Configuration that GitProject or
+    // LocalProject returned; nullopt for any other: the project crafter-build
+    // was pointed at, and every configuration a project file makes itself.
+    std::optional<LoadedProject> LoadedProjectOf(const Configuration& cfg);
+}